Physical security for buildings, offices, residences, etc. is a growing concern. One such security concern is damage caused by explosions, such as a bomb detonation, that may occur exterior to a structure. Though a building's inherent structural integrity can often mitigate the impact of some types of explosions, the impact can be aggravated by the presence of windows in the building. Glass shards from breaking windows can cause substantial damage and injury to persons and property inside a building even if the structural damage to the building is minimal. It has been reported that glass shrapnel from shattering windows causes over eighty percent (80%) of the serious injuries from a bomb blast event.
Many useful devices have been developed to secure and protect structures from blast events. These devices can be divided into two broad categories: (i) replacement of the existing glass and framing window system with a blast resistant window system, and (ii) installation of a retrofit product or products onto or in front of the existing glass and framing system on the interior of the building, while keeping the original window unit in place.
Typically, the most effective method is to completely replace the existing window system with a blast resistant window system designed specifically for the building's structure and the estimated blast load; however, it can be cost prohibitive to treat an entire building in this manner. Another option is to install retrofit products such as fragment retention films that can be anchored to the existing window frame; however this approach has its own limitations and may not be a viable option for many reasons, such as, for example: (i) hardening the window with current retrofit treatments may actually cause greater structural damage to the building in a blast event; (ii) the window, glass, or frame construction may not allow hardening using current retrofit treatments; or (iii) the available retrofit treatments that are technically possible are not aesthetically acceptable.
The typical minimum protection technique for retrofitting windows is to apply a fragment retention film (FRF) or shatter-resistant window film (SRWF) (collectively, “blast protection film”) to the visible portion of the glass in what is termed a “daylight configuration.” Although the fragment retention film will hold the glass shards together during a blast event, the window pane may fly into the room as one piece, possibly causing blunt trauma injury.
The fragment retention film can be anchored to the existing window frame using various techniques. This application usually is sufficient for low level blasts if the existing window frame has sufficient structural integrity to accept the blast load generated by the film and anchoring system. In some window systems, however, it is not feasible to install an anchored fragment retention film. Therefore other retrofit fragment retention film configurations must be used in conjunction with products that catch the filmed glass after it leaves the window frame in a blast event.
In the mid 1990s, the US Army Corps of Engineers developed a retrofit “catchbar system” that consisted of a steel tube placed across a window and mounted securely into the structure's wall. The window glass was treated with a fragment retention film. During a blast, the bar literally caught the filmed glass as it exited the window frame. The US Army Corps of Engineers published its blast results and design in an Engineering Technical Letter for use by manufacturers, designers, and end users describing the design and implementation of this concept. This method worked well for lower blast pressures, but at higher blast pressures the film tore from impact with the rigid catch bar, allowing two pieces of filmed glass to fly into the room.
A solution to this problem was developed in the form of a deployable catchbar. The deployable catchbar system consists of a catchbar which contains and conceals a steel cable that is fastened on each end to the window frame or to the building's structure as appropriate. In a blast, the filmed glass is blown into the catchbar, which is mounted an inch or two from the glass window on the inside of the building. The catchbar engages the glass and exits the frame as well; however, the steel cable allows the catchbar to travel a short set distance but then stops the catchbar and glass sheet from traveling any further. The advantage of this system is that it allows the blast pressure to vent around the glass sheet and decelerates the glass sheet less abruptly. The assignee of the present application blast tested this product and presented its results to the Protective Glazing Council Symposium in 2000 at the General Services Administration (GSA) Headquarters Building in Washington, D.C. Many variations of this product have been developed and sold by different manufacturers. Some manufacturers even use a cable or strap system without the catchbar depending on the design blast load and aesthetics.
The catchbar concept, while effective, does has some drawbacks. Its effectiveness depends on the number of catchbars mounted across the window, and even the deployable version may cause the filmed glass to split where the catchbar engages the filmed glass in large blasts, at least with bare cable catchbars. Also, this approach is relatively ineffective when used in conjunction with insulating glass since only the interior pane is treated with fragment retention film.
U.S. Published Application No. 2006/0032160A1 to Gazaway et al. entitled “Retrofit Glass Fragment Catching System” describes a blast resistant window blind system, the entirety of which is hereby incorporated by reference herein.
An improved blast resistant window system is desired.